Unmasking of Ventricular Preexcitation

Twenty-one patients were studied in whom ventricular preexcitation (VP) had been recorded in the past and had later disappeared, indicating antegrade block in the accessory pathway (AP), either spontaneously (10 patients) or under the effect of chronic treatment with amiodarone (11 patients). VP reappeared in nine cases during vagal stimulation, and in five cases during an i.v. isoproterenol infusion. Retrograde conduction over the AP was studied in four of the remaining seven patients and was found to be present in three and absent in one. Although these patients differ from the ordinary patient with concealed AP in that antegrade preexcitation had been demonstrated in the past, this study suggests that concealed VP may result from the following mechanisms: 1) an extremely prolonged refractory period in the AP, causing a ratedependent VP that can be identified during vagal stimulation; 2) a rate-independent depression of antegrade conduction that can be reversed by isoproterenol; 3) a depression of conduction that is apparently no longer